This report introduces a novel UV-curing technology based on thiol-thiol coupling for polydisulfide network formation. Beginning with a model tris(3-mercaptopropionate) trithiol monomer and xanthone propionic acid-protected guanidine as photobase generator, a comprehensive characterization based on spectroscopic techniques supports the reaction of thiols into disulfides without side reactions. The best experimental conditions are described as regards to film thickness, irradiance, emission wavelength, and atmosphere composition. The results shed light on a step-growth photopolymerization mechanism involving two steps: first, the formation of thiyl radicals by thiolate air oxidation or/and thiol photolysis, and second, their recombination into disulfide. By varying thiol V C 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 117-128.

Initially developed as an elastomer with an excellent record of barrier and chemical resistance properties, poly(disulfide) has experienced a revival linked to the dynamic nature of the S-S covalent bond. A novel photobase-catalyzed oxidative polymerization of multifunctional thiols to poly(disulfide) network is reported. Based solely on air oxidation, the single-step process is triggered by the photodecarboxylation of a xanthone acetic acid liberating a strong bicyclic guanidine base. Starting with a 1 μm thick film based on trithiol poly(ethylene oxide) oligomer, the UV-mediated oxidation of thiols to disulfides occurs in a matter of minutes both selectively, i.e., without overoxidation, and quantitatively as assessed by a range of spectroscopic techniques. Thiolate formation and film thickness determine the reaction rates and yield. Spatial control of the photopolymerization serves to generate robust micropatterns, while the reductive cleavage of S-S bridges allows the recycling of 40% of the initial thiol groups.

A UV photoinduced cross-linking of non-modified commercial poly(disulfide) resins (Thioplast) is reported via the air oxidative photocoupling of terminal thiol functions. Catalyzed by a photogenerated guanidine base (TBD), this step-growth photopolymerization is useful to maximize disulfide functions content. The mechanism proceeds through thiol deprotonation into thiolate anions, further oxidized into thiyl radicals, eventually dimerizing into disulfide cross-links. Starting with a detailed structural characterization of the thiol-terminated resin, photooxidative kinetics are studied under exposure to a polychromatic medium-pressure Hg arc using Raman and infrared spectroscopy. The effects of irradiance, film thickness, photobase concentration, resin molar mass, and content of an additional polythiol monomer (reactive diluent) have been investigated. In an effort of upscaling, irradiation under a 365 nm LED panel has enabled the fast preparation of 1.5 µm thick cross-linked poly(disulfide) coatings in a matter of minutes. Capitalizing on the ability of residual thiol groups to react with silver cations, a post-functionalization has been successfully performed, leading to films exhibiting at their surface stable thiolate-silver bonds as proved by X-ray photoelectron spectroscopy. Despite the well-established biocide action of silver ions, no antibacterial action has been evidenced by confocal fluorescence microscopy because of insufficient release.

Base catalyst and oxidant are usually necessary to promote the polymerization of poly(disulfide) oligomers through oxidative coupling of the terminal SH groups into S–S bonds. In this study, we prove that self-polymerization of bifunctional (disulfide) oligomer films can take place in a matter of minutes under UVC irradiation (254 nm, 10.5 mW cm –2 ). The resulting insoluble polymer is characterized using solid-state NMR, 1 H T 2 NMR relaxation measurements, thermal analysis, and Fourier-transform infrared spectroscopy and proves to have similar composition as a model poly(disulfide) prepared under oxidative conditions, but distinct physical properties. These differences are explained by a change in polymer architecture due to a higher ratio of cyclization relative to linear polymerization. Homolytic photocleavage of internal S–S bonds creates thiyl groups close to each other, driving an increased kinetic feasibility for the cyclization reaction by radical coupling. The subsequent formation of mechanically interlocked macrocycles (polycatenane network) is proposed to account for film properties analogous to those of a cross-linked polymer.